Metal has long been melted by many useful techniques including batch processing techniques, in which the melt is poured in discrete batches, and by continuous techniques. The melting energy in the known systems is provided by various techniques such as induction, electric arc, gas, and energy beams.
One melting technique, known as skull melting, utilizes a hearth cavity commonly heated by a radiant beam, such as, for example, an electron beam. In skull melting, a certain portion of the melted metal is allowed to freeze in the hearth cavity to form a lining or skull along the inner surface of the hearth cavity because the melting temperature of the metal being melted may be higher than the melting temperature of the hearth and also the metals are frequently too reactive to be in contact with other substances. In skull melting, the metal being poured is then contained in the hearth or vessel and skull of frozen material of the same metal. Thus to avoid the formation of, for example, undesirable oxides or other side products, the metals are melted in and poured from a skull of the same material, and often under an inert atmosphere, such as, but not limited to, nitrogen, helium, or argon. The molten metal is then poured out of the skull cavity, often onto a ribbon making or strip making or filament making device.
It is desirable to have the skull as thin as possible to minimize the unused metal and improve pour efficiency. However, it is often difficult to control the thickness of the solidified skull. It is also difficult to control the uniformity of the cooling around the perimeter of the skull, and thus the rate of freezing of the skull sections. Conventionally, the radiant beam is repositioned frequently to attempt to melt or remelt the metal being cast, but such spot heating does not lead to uniform heating or cooling and can result in poor melting efficiency.
In addition, the continued freezing of more molten metal reduces the efficiency of the skull casting technique to the point that it is common for up to about 80% of the metal contained in the hearth to ultimately remain frozen as the skull.
U.S. Pat. No. 4,469,162, issued Sep. 4, 1984 to Hanas et al., teaches the use of a ladle with a temperature sensor and control of the heat input to the ladle.
U.S. Reissue Pat. No. 27,945, issued Mar. 26, 1974 to Hunt et al., teaches the use of a skull-type system in which the observation is made that the skull thickness is dependent upon heat removal which can be regulated to control the desired thickness of the cast part.
U.S. Pat. No. 4,674,556, issued June 23, 1987 to Sakaguchi et al., teaches the supply of water to a roller and a temperature detector which is used to control the supply rate of cooling water in non-skull melting systems.
U.S. Pat. No. 4,483,387, issued Nov. 20, 1984 to Chieler's et al., teaches the use of a continuous casting mold in which the temperatures of different sections of the mold are independently controlled by calculating the quantity of cooling water delivered to each of the sections.
Thus a need exists for a method and apparatus for the controlled heating and cooling of skull melting whereby the thickness of the solidified skull and the width and depth of the molten material can be easily adjusted. It is desirable to control the skull thickness and to adjust the temperature without causing the mold or hearth to melt, degrade, or deform. It is also desirable to minimize the thickness of the skull to thereby increase the effiency of the skull melting process by allowing more of the castable metal to obtain, a molten state.